Liquid crystal displays have been enjoying great popularity. Taking advantage of their features-slimness, low weight, and low power consumption, they are achieving rapidly growing use in various fields, not to mention mobile applications.
Mobile phone applications are especially notable. The slimness, low weight, and low power consumption features have been enhanced. On top of that, the liquid crystal display now boasts excellent visibility owing to improvements in high definition technology. These factors are fueling explosive and widespread popularity of the liquid crystal device in the field.
Improvements have been also made on viewing angle expansion films and in wide viewing angle technology based on the use of vertical alignment mode and IPS (in-plane switching) alignment mode. The achievements are applied in, among others, those fields where two or more people are expected to watch the screen together: for example, car navigation systems, notebook computers, monitors, and liquid crystal televisions. A recent trend is the rapidly growing use of the large screen liquid crystal television.
It is undoubtedly a benefit of the wide viewing angle technology that the image on the liquid crystal display has become viewable by many people. Meanwhile, current development in IT technology has enabled the user to obtain and view various information at various public places. People other than the user can view graphic information on the image display primarily made from the liquid crystal display. The protection of privacy over the image is a serious issue. Solutions must be offered as soon as possible.
The following will describe conventional liquid crystal displays. A typical liquid crystal display includes a pair of transparent electrode substrates positioned face to face. Pixel electrodes are provided in a matrix. Also provided are active elements such as thin film transistors which are switching means selecting the pixel electrodes which apply voltage to liquid crystal.
In the space between the pair of transparent electrode substrates, a liquid crystal layer is formed. Conventionally used liquid crystal layer alignment modes are twisted nematic alignment mode (“TN”) and super twisted nematic alignment mode (“STN”). In addition, vertical alignment mode (“VA”) and in-plane switching (“IPS”) are widely employed in recent liquid crystal displays with very wide viewing angles.
Incidentally, the aforementioned liquid crystal displays, especially, those with very wide viewing angles employing VA mode or IPS mode, have very wide viewing angles as shown in FIG. 11. Images are recognizable to viewers B other than the primary viewer A who is right in front of the liquid crystal display plane. The displays are suitable for many viewers watching together.
However, the wide viewing angle of the liquid crystal display poses a problem: the other viewers B can view the graphic information that the primary viewer A does not want the other viewers B to view. To provide only the primary viewer A with desired graphic information while preventing the other viewers B from peeking at the information, the viewing angles need be narrowed down deliberately.
The viewing angle mode needs be switched between this wide viewing angle mode for many viewers watching together and a narrow viewing angle mode where the viewing angles are narrow for the purpose of protecting privacy, depending on the needs of the user of the liquid crystal display. Wide viewing angle mode and narrow viewing angle mode need be switchably implemented on the same display.
In an attempt to address the problem, Japanese published patent application 9-105907/1997 (Tokukaihei 9-105907; published on Apr. 22, 1997) discloses a technique for the liquid crystal display of which the viewing angle is controllable. The liquid crystal display is capable of switchably implementing wide viewing angle mode and narrow viewing angle mode.
Next, the disclosed technique will be briefly described.
FIG. 12 and FIG. 13 show schematic drawings of the viewing-angle-controllable liquid crystal display. In the figures, there is provided an image display 13 displaying desired graphic information to viewers. The image display 13 includes multiple pixels arranged in a matrix. Each pixel is capable of individually modulating/controlling light from a backlight unit 11 placed behind the image display 13.
In the space between the image display 13 and the backlight unit 11 is there provided an optical element 90 controlling the viewing angle. The optical element 90 includes a pair of transparent electrode substrates positioned face to face. The space between the substrates is filled with a polymer-dispersed liquid crystal. The liquid crystal scatters or transmits light from backlight 11 according to electric signals. The scattering is controlled by means of voltage applied to the pair of transparent electrodes of the optical element 90.
The backlight unit 11 is adapted so that its brightness can be set to a given level through control of voltage applied to a cold cathode fluorescent tube which is a part of the unit.
First, the low viewing angle mode will be described in reference to FIG. 12. In the mode, the brightness of the backlight unit 11 is lowered by reducing the power supply to the backlight unit 11. Simultaneously, the drive voltage for the optical element 90 is set to a predetermined value, so that the polymer-dispersed liquid crystal does not scatter any light at all.
In this situation, light from the backlight unit 11 passes through the optical element 90 as shown in the figure. The scattering property of the transmitted light does not change at the optical element 90. The light leaves the image display 13 within the angle θn as in the figure. The narrow viewing angle of the viewing-angle-controllable liquid crystal display is substantially equal to the viewing angle of the image display 13.
When other viewers are to be blocked from the graphic information before the viewing-angle-controllable liquid crystal display, there is no need for a wide viewing angle. The purpose is purportedly achieved by the use of the liquid crystal display in the narrow viewing angle mode.
Next, the wide viewing angle mode will be described in reference to FIG. 13.
In the wide viewing angle mode, the brightness of the backlight unit 11 is raised by increasing the power supply to the backlight unit 11. Simultaneously, the drive voltage for the optical element 90 is set to a lower value than the predetermined value, so that the polymer-dispersed liquid crystal scatters increased quantities of light.
In this situation, light from the backlight unit 11 is scattered by the polymer-dispersed liquid crystal in the optical element 90 as shown in the figure. The light passes through and leaves the image display 13 within the angle θw in the figure. The light from the backlight 11 reaches the image display 13 after being diffused by the optical element 90. The viewing angle of the viewing-angle-controllable liquid crystal display is greater than the viewing angle of the image display 13.
When two or more people are to watch the liquid crystal display together, the liquid crystal display needs to exhibit a wide viewing angle. This wide viewing angle mode is hence useful.
Controlling the optical scattering of the optical element 90 which contains a polymer-dispersed liquid crystal as a viewing-angle-controlling element by means of drive voltage in this manner can purportedly achieve switching between the narrow viewing angle mode and the wide viewing angle mode.
In the technique described in the document, the polymer-dispersed liquid crystal is provided across the entire display area. The optical scattering is regulated through application voltage to regulate the viewing angle.
In the narrow viewing angle mode, a predetermined voltage is applied to the optical element containing a polymer-dispersed liquid crystal to prevent the scattering of light from the backlight so that the viewing angle of the image display element is used for the narrow viewing angle as it is. It is impossible to narrow the viewing angle of the image display.
Current mainstream liquid crystal modes are vertical alignment, IPS, and other modes where the viewing angles are very wide. This is especially true from large- to medium-size notebook computers and monitors to mobile applications. In the foregoing technology whereby the viewing angles in narrow viewing angle mode correspond to the viewing angles of a liquid crystal panel for image displays, the narrow viewing angle mode can no longer be achieved.
Further, in the wide viewing angle mode, the optical scattering by the optical element 90 containing a polymer-dispersed liquid crystal is exploited to expand the viewing angles. Therefore, if the optical scattering is increased for viewing angle expansion, the reflectance of the optical element for the light from the backlight unit increases, and the transmittance decreases. The falling transmittance in turn decreases the brightness of a liquid crystal panel used for display purposes. Image quality thus drops.
In addition, when viewing displays on a liquid crystal display element in typical situations, a use environment where an image with wide viewing angle characteristics is used as an ordinary screen and the narrow viewing angle mode is used according to a situation is considered common. It is therefore a problem for the falling brightness to degrade image quality in the wide viewing angle mode as described above.